1. Field of the Invention
The present invention relates to packaging substrates and fabrication methods thereof, and more particularly, to a packaging substrate for solder bumps to be formed thereon and a fabrication method thereof.
2. Description of Related Art
In a conventional flip-chip semiconductor package, a plurality of solder bumps are formed on the conductive pads of a semiconductor chip and electrically connected to a packaging substrate so as to electrically connect the semiconductor chip to the packaging substrate. Compared with wire bonding, the flip-chip technology provides shorter electrical path and better electrical performance. Further, the inactive surface of the semiconductor chip can be exposed from the package structure so as to improve the heat dissipating efficiency.
FIG. 1 is a schematic cross-sectional view showing a packaging substrate having a UBM (under bump metallurgy) layer as disclosed by U.S. Pat. No. 5,937,320. Referring to FIG. 1, a titanium layer 11a and a copper layer 11b are formed on the conductive pads 101 of a semiconductor chip 10 first and then solder bumps 12 are formed on the copper layer 11b corresponding in position to the conductive pads 101. Thereafter, portions of the titanium layer 11a and the copper layer 11b that are uncovered by the solder bumps 12 are removed by etching so as to define a UBM layer 11 under each of the solder bumps 12. As such, the solder bumps 12 are securely attached to the conductive pads 101 of the semiconductor chip 10 through the UBM layer 11.
However, during the etching process, since the titanium layer 11a is etched faster than the copper layer 11b, it results in serious side-etching of the titanium layer 11a. As such, a significant undercut structure is formed as shown in FIG. 1. The undercut structure induces stress concentration such that the overall solder bump structure easily cracks at the stress concentration point, thereby reducing the product reliability.
Therefore, there is a need to provide a packaging substrate and a fabrication method thereof such that the significant undercut structure caused by serious side-etching of the UBM layer can be avoided so as to increase the product reliability and yield.